The invention relates to receivers for optical telecommunications applications, and more particularly to silicon optical benches used therein.
Silicon optical benches typically comprise a single crystal semiconductor such as silicon as a base material upon or within which an optoelectronic device subassembly is constructed. Silicon optical benches may be used to fabricate receiver modules for optical telecommunications. As depicted in FIG. 1 a receiver module may include a photodetector 102 and a mirror 104 to direct incident radiation from an optical fiber 106 onto the photodetector active region 108. Typically, the incident light is focused onto mirror 104 by a coupling lens 110. The bench facilitates the alignment of light from fiber 106 such that it focuses on mirror 104. Mirrors may be formed directly in the silicon bench by etching along the (1,1,1) plane and coating the resulting exposed face or faces with aluminum or gold. The etched planes for silicon are at an angle of 35.7xc2x0 to the incident light, which results in a reflection which is 108.6xc2x0 from the optical axis. The optimum reflection angle, however, for directing incident light onto photodetector active region 108 is typically 90xc2x0. Accordingly, light incident on photodetector 102 is at an 18.4xc2x0 angle relative to the surface normal, and therefore, may miss active region 108.
As shown in FIG. 2, when a lensed photodetector 202 is used, a focus spot 204 is formed away from the center of the active photodetector region 206. In this configuration 60% to 75% of the focused light may miss active region 206. This may lead to decreased coupling efficiency and lower overall signal levels on the receiver.
A lens may be formed on the photodetector surface to concentrate the light into a smaller area, and thereby increase the coupling efficiency. The disadvantage of this approach is that some or all of the light is likely to be incident outside of the active area of the photodetector when a mirror etched on the silicon bench is used.
Increasing the active region of the photodetector may compensate for alignment problems caused by mirrors etched in the silicon (1,1,1) plane. This, however, has the undesirable effect of increasing device capacitance, and therefore, reducing the usable bandwidth of the device due to the increase response time constant. This may also lead to a need for larger devices to generate the same photocurrent. As device size increases, fewer devices may be fabricated from a single wafer, thereby increasing the cost per device.
Accordingly, there is a need to accurately focus incident radiation on the active region of a photodetector in an optical subassembly bench when a mirror position in the subassembly is dictated by the crystal structure of the bench material.
An optical lens for use with an optical bench is disclosed. The lens consists of a diffractive element which provides an angular offset to radiation incident on the lens while allowing it to be focused to a point. The angular offset substantially compensates for an undesirable focal point location caused by a variance between an integral component position on the bench and a desired position. The lens is particularly useful wherein an integral component position is dictated by the bench crystal structure. In an illustrative embodiment the diffractive element is aspheric and deflects incident radiation by an amount in the range of about 8xc2x0 to about 12xc2x0 to compensate for the focal point location produced by an integral component positioned on a silicon (1,1,1) plane.
Further disclosed is a method of compensation for variance between an integral component position on a bench and a desired position. Still further disclosed are an optical bench. a method for fabricating an optical bench and a semiconductor device.